Photoelectrochemical cell

ABSTRACT

A method for converting solar energy to electricity is provided using solid-liquid interface photoelectrochemical cells wherein the liquid phase comprises a nonaqueous solvent, an electrolyte dissolved therein forming an ionically conductive solution and a redox couple suitable to accept and donate electrons from and to the electrodes. The redox couple is present in an amount sufficient to sustain a predetermined current and the concentrations of the electrolyte and redox couple in the solution are sufficient to provide no greater than a selected small voltage drop relative to the output voltage of the cell. The efficiency of conversion of light to electrical energy of such photoelectrochemical cells is 10% and greater.

This is a continuation-in-part of copending Ser. No. 451,336, filed Dec.20, 1982, now U.S. Pat. No. 4,459,343, issued July 10, 1984.

The present invention relates to photoelectrochemical cells, and inparticular, to nonaqueous liquid junction cells useful as solar cells.

In a cell utilizing a semiconductor-liquid junction, the liquid is anionically conductive material. The analogy of the charge transferphenomenon at the junction of the liquid with a solid semiconductor isthe solid state Schottky barrier. In a semiconductor-liquid junction,the liquid plays the role of the metal overlayer in a classical Schottkybarrier system. For example, if an n-type semiconductor is placed incontact with a liquid solution containing an electroactive species(i.e., a chemical redox couple) such that the redox potential of theelectroactive species (the Fermi level) is more positive relative tovacuum than the conduction band of the semiconductor, charge transferwill take place until the equilibrium situation has resulted in a spacecharge layer of appreciable thickness (depletion region) in thesemiconductor. The width of this depletion region, by analogy to theSchottky barrier systems, is related to the amount of band bending, thedopant density of the semiconductor and the dielectric constant of thesemiconductor. The field created in the depletion region may be of aphysical dimension such that it will be effective in separatingphotogenerated electron-hole pairs created by light absorption at thesemiconductor-liquid interface. Therefore, if the semiconductor is ann-type, for example, photogenerated holes will be driven by the spacecharge field to the semiconductor surface toward the solid-liquidinterface. The electrons will be driven into the bulk of thesemiconductor. The holes will be consumed by electron donation from theelectroactive species in the liquid (oxidation of the redox species),while the electrons can be withdrawn from the semiconductor, passedthrough a load, and reinjected into the liquid. If the injection of theelectrons back into the liquid results by reduction of the redox speciesin the production of the same chemical species which was used to quenchthe photogenerated hole, then there will be no net chemical change inthe liquid and the overall conversion will be light to electrical power.Depending upon the electroactive species present in the liquid,electrons may produce a different chemical species from that used toquench the photogenerated holes, in which case the cell may function asa battery in storing energy for later use.

It is therefore an object of the present invention to providesemiconductor-liquid junction photoelectrochemical cells which haveimproved efficiency.

It is a further object of the present invention to provide improvedsolar cells having improved solar to electrical energy efficiency,usually in the range of 10% and above.

It is a further object of the present invention to provide thin layerliquid junction cells having improved solar-to-electrical energyconversions.

These and other objects will become apparent from the followingdescriptions and claims.

In the accompanying drawings:

FIG. 1 shows the band diagram which characterizes a semiconductor-liquidjunction.

FIG. 2 is an elevational view of a semiconductor liquid junctionphotoelectrochemical cell.

FIG. 3 shows the band diagram which characterizes aliquid-insulator-semiconductor (LIS) junction.

FIG. 4 shows the plot of current v. potential of a 3-electrodephotoelectrochemical cell according to Example 1.

FIG. 5 shows the plot of current v. potential of a 2-electrodephotoelectrochemical cell according to Example 2.

FIG. 6 is a plot of quantum efficiency v. photon energy of a 2-electrodeLIS cell according to Example 2.

FIG. 7 is a plot of power curve v. time of a 2-electrode LIS cellaccording to Example 7.

FIG. 8 shows the plot of current v. potential of a 2-electrode LIS cellaccording to Example 8.

FIG. 9 illustrates the results of the stability test described inExample 10.

The present invention is directed to semiconductor liquid junctionphotoelectrochemical cells containing a nonaqueous solvent and anelectrolyte which form an ionically conductive solution, and furthercontaining a redox couple dissolved in the solution which is suitable toaccept and donate electrons from and to the electrodes of the cell uponexposure of the cell by an external source of light. We have discoveredthat the redox couple should be present in an amount sufficient tosustain a predetermined current, and that the concentrations of theelectrolyte and the redox couple in the solution must provide a smallvoltage drop compared to the output voltage. We have discovered furtherthat by limiting the total water content in the solution, aliquid-insulator-semiconductor junction (LIS) may be formed whichprovides a photoelectrochemical cell which does not degradesignificantly over time. We have also discovered that by providing athin liquid layer in a liquid junction cell, improved efficiency may beobtained without imposing mechanical stirring since diffusive conductionwill be sufficient to conduct the charge.

According to the present invention, the efficiency of thephotoelectrochemical cell may be improved by one or more of acombination of factors, including, minimization of the ohmic losses dueto the liquid and solute components of the cell, and improvement of themass and charge transfer processes at the solid liquid interface and inthe solution.

The interface energetics, or band diagram, for an idealsemiconductor-liquid interface where the liquid contains a redox coupleat charge transfer equilibrium is shown in FIG. 1. The vertical linerepresents the semiconductor electrolyte interface. The valence bandedge and the conduction band edge of the semiconductor are denoted byE_(VB) and E_(CB), respectively. The band gap is indicated by the arrowE_(BG), the potential of the redox couple by E_(Redox) and thesemiconductor Fermi level by E_(F). The presence of a depletion layer inthe semiconductor results in the separation of photogeneratedelectron-hole pairs (e⁻, h⁺), with the electrons being driven into thebulk (as shown, for an n-type semiconductor) and the holes migrating tothe semiconductor/liquid interface. The holes are consumed by a solutionreductant, resulting in the flow of current. The voltage differencebetween the energy at the edge of the semiconductor conduction band,E_(CB), and the redox potential, E_(Redox), determines the barrierheight of the cell. The band gap for semiconductor materials in solarcells is preferably 1.0 to 2.3 volts.

The band diagram for the LIS junction is shown in FIG. 3 for an n-typesemiconductor. The insulator region may be a limited thickness,deliberately formed oxide layer resulting from the limitation of thetotal water content of the solution. The presence of such an oxideinsulator layer may improve the short circuit current, I_(sc), and opencircuit voltage, V_(oc), of the cell by acting as a barrier to majoritycarriers. The formal analysis of an LIS structure is analogous to thesolid state treatment of an MIS structure.

The photoelectrochemical cells according to the present inventioncomprise a semiconductor photoactive electrode, a nonaqueous solventcontaining a dissolved electrolyte forming an ionically conductivesolution, an electrode providing an electrical connection from thesolution to the photoactive electrode, and a redox couple dissolved inthe solution which is suitable to accept and donate electrons from andto the electrodes upon exposure of the cell to an external source oflight, such as solar radiation.

The semiconductor photoactive electrode may comprise any semiconductormaterial, either of the n-type or the p-type, which may be singlecrystal, polycrystalline, or amorphous. For example, a semiconductormaterial may be silicon, gallium arsenide, gallium arsenide phosphide,cadmium telluride, cadmium selenide, indium phosphide, or a-Si:H alloy.preferred semiconductor materials are silicon, gallium arsenide andgallium arsenide_(x) phosphide.sub.(1-x), wherein x is between 0.0 and1.0.

The solvents used in the liquid phase of the semiconductor liquidphotoelectrochemical cells according to the present invention arenonaqueous. The nonaqueous solvent may be of the type whichself-dissociates into solvent molecules which are ionically conductiveor may be a solvent such that an electrolyte added thereto will besubstantially dissociated to form an ionically conductive solution. Toobtain optimum efficiencies of the cell, it is preferable that thesolvent have a relatively high dielectric constant in order to achievesubstantial dissociation of the electrolyte. Also, it is preferable thatthe solvent have a relatively low viscosity in order to facilitate masstransfer of the electrolyte ions through the liquid. Therefore,preferred solvents should have dielectric constants greater than about20 and viscosities less than about 2 centipoise. A preferred class ofsolvents comprises the alkanols of 1 to 10 carbon atoms, particularlymethanol. A second preferred class of solvents comprises nitriles of 2to 10 carbon atoms, particularly acetonitrile. The most preferred classof solvents comprises a mixture of alkylene carbonate, such as propylenecarbonate, with a minor amount of an alcohol. The alcohol in such amixture may be a linear or branched, unsubstituted or halogenatedalcohol of 1 to 10 carbon atoms. Exemplary alcohols include n-octanol,n-hexanol, n-butanol, trifluoroethanol, and methanol. The amount ofalcohol used in the mixture will usually be less than 50% by weight ofthe alkylene carbonate: alcohol mixture, preferably 15% or less. Due toits availability and low vapor pressure, n-octanol is preferred.

Generally, the solvent will be a matter of choice within the ordinaryskill of those in the art, utilizing conventional tables of solventviscosities and dielectric constants, such as, ExperimentalElectrochemistry for Chemists, Sawyer, Donald T., and Roberts, JulianL., Jr., John Wiley & Sons, New York (1974), Chapter IV. While thesolvents utilized in accordance with the present invention are to benonaqueous, conventional means of drying solvents may be utilized, withthe realization that, in many instances, the solvent will not beabsolutely free of water even after treatment with the drying agent orconditions. In the case of a silicon semiconductor, the reason for theremoval of undesirable excess water from the solvent is to decrease theopportunity for electrode decomposition or passivation processes, whichpredominantly occur by oxide growth from the reaction of thesemiconductor with residual water. If the oxide layer is too thick, itwill completely insulate the semiconductor from the solution.

There is therefore a limit on the water content which should bepermitted in the solvent in order to obtain long term operation of thecell without significant occurrence of passivation reactions involvingwater as a reactant. The number of permissible water molecules, n, in agiven volume of solution, V, to insure that there is insufficient waterpresent to form an insulating oxide layer on a semiconductor may becalculated. If there are 10¹⁵ molecules per centimeter square on theelectrode surface, formation of 40 Å of oxide, or about 8 monolayers,would require 8×10¹⁵ molecules of water per square centimeter of exposedelectrode area. Assuming a rectangular geometry for the cell, either thethickness of the liquid, t, or the concentration of water, M, can bedetermined if the other is given, according to the following equation:##EQU1## This equation should be general for any given volume andgeometry of cell. Since according to conventional methods, it isunlikely that the solvent could be dried of water below about 0.01 ppm(10⁻⁸ moles/liter or 6.02×10¹⁷ molecules per liter), this places anupper limit on the volume of the nonaqueous solvent permitted to be incontact with the semiconductor. The restricted amount of water presentin the solvent is sufficient to form a limited amount of oxide to createan LIS junction as shown in FIG. 3. Typically, the power output of atwo-electrode LIS cell will not degrade substantially over time. For asilicon semiconductor, an oxide layer in the range of 5-40Å is preferredfor the LIS junction.

The electrolytes utilized in accordance with the present inventionshould be of the type which are substantially dissociated in the solventto form an ionically conductive solution. Furthermore, the amount ofelectrolyte present is to be selected in an amount sufficient to provideno greater than a predetermined maximum electrical resistance due to thesolution. The resistance of the nonaqueous solution, including thesolutes dissolved therein, to current flow should be minimized in orderto prevent large resistive losses. The electrolyte must thereforedissolve in the solvent and dissociate to yield conductive ions and mustresult in conductivities large enough to meet the resistivityspecifications at the given concentration. Therefore, to predeterminethe maximum desired resistance for the solution the following equationis utilized:

    R=ρ1/A

where R is resistance and p is the resistivity in ohm-cm. For example,at solar current densities, about 50 mA/cm², the voltage loss, IR, dueto any particular solution resistance may be calculated. A typical cellmay have an output voltage of around 0.50 volts. A predetermined limitfor IR losses due to the solution may be selected to be 10% or less.Therefore, if the voltage loss is 0.05V, and the current is 0.05A/cm² Rshould be 1 ohm or less for a 1 cm² area. Use of the above equationrelating R to solution thickness, 1, results in the limit that 1ρ=1 willinsure that the solution resistance will not be more than 10% of the 0.5volt output voltage. Therefore if a cell thickness is specified, at 0.1cm, the requirement would be a ρ of less than 10 ohm-cm, whichcorresponds to a conductivity of greater than 100 mmho cm⁻¹. To meetthese predetermined requirements, conventional tables of conductivitiesof ions in various solvents may be utilized, such as Conductance ofElectrolytes in Organic Solvents, Kratochvil, B., and Yeager Howard L.,Chem. Forsch., Vol. 27, p. 1 (1972).

In methanol, a preferred electrolyte according to the present inventionis lithium perchlorate because of its solubility. In acetonitrile, thepreferred electrolytes are the quaternary ammonium salts, particularlyquaternary ammonium borontetrafluoride salts. A preferred electrolyte istetraethyl ammonium borontetrafluoride.

The electroactive species, or redox couple, dissolved in the liquid inaccordance with the present invention should have a redox potential suchthat it is suitable to accept and donate electrons from and to theelectrodes in the cell upon exposure of the cell to an external sourceof light. The redox couple should be present in an amount sufficient tosustain a predetermined maximum current flux relative to the maximumphoton flux of the incident light. Also, as a solute in the liquid, theredox couple should meet the limitations as to imparting no greater thana predetermined maximum electrical resistance due to the solution.However, in most circumstances the electrical resistance of the solutionshould be due primarily to the electrolyte and it will be normallysufficient to consider the current flux capability of the redox coupleon the presumption that the solution resistance requirement can besatisfied by adjustment of the electrolyte concentration. During theoperation of the cell there must be a steady state flux of redoxmolecules to the photoactive electrode surface in order to quench thephotogenerated holes (in the case of an n-type semiconductor) or toquench the photogenerated electrons (in the case of a p-typesemiconductor). The flux of redox molecules will depend upon whethernatural diffusive convection is utilized in the solution or whether thisis enhanced by agitation such as by thermal stirring, mechanicalstirring, ultrasonic stirring and the like.

In a preferred embodiment of the present invention, the solution will bepresent in the cell in a thin layer, preferably having a thickness lessthan 100 microns, usually in the range of 10 to 100 microns. A cellhaving a thin solution layer is advantageous since the liquid volumebetween the electrodes is relatively small, therefore giving a higherpermissible water concentration, wherein the limit of water present isdetermined as described above in connection with the formation of an LISjunction. Furthermore, the predetermined maximum permissible resistanceof the solution may be satisfied by a thin layer, given that electrolyteconductivities are usually less than 100 mmho cm⁻¹. Additionally,mechanical stirring of a thin layer is not required since naturaldiffusion and thermal stirring caused by impingement of solar radiationon the solution will usually be sufficient. The rate of diffusion ischaracterized by the diffusion coefficient, D, of a molecular species ina particular solvent. The parameter D is typically 10⁶ cm² per second.In a thin cell, the relationship between the thickness of the solutionand the concentration of redox species may be shown according to thetheory of Bard, et al., Electrochemical Methods, Bard, A. J., andFalkner, L. F. John Wiley & Sons, N.Y., 1980, by the following formula:

    i.sub.D =2FDC.sub.0 /t

(F=9.65×10⁴). Therefore, for a typical terrestrial sunlight intensity,the current density, i_(D), is expected to be no greater than 50 mA/cm²for materials with band gaps greater than 1.0 eV. Assuming D=10⁻⁶ cm²per second, the requirement is therefore that the ratio C₀(concentration of electroactive species) to t (thickness)=260 or more inorder to support the current flux by only natural diffusion. Therefore,if t is chosen to be 10 microns, then 0.25M of electroactive specieswill meet the current density requirement. In systems where t is muchlarger, either higher concentrations of electroactive species or forcedconvection will normally be utilized. Generally the current densitiesavailable from redox solutions assisted by mechanical stirring are inthe order of 0.5 A/cm² at 1M concentration electroactive species.Therefore, to achieve greater than 50 mA/cm² it would require greaterthen 0.1M of electroactive species dissolved in a solution withefficient stirring. Since the current density requirement relates toboth electrode surfaces, i.e., the surface of the semiconductorphotoactive electrode and on the electrode providing a connection fromthe liquid to the load, the solubility limits of both the oxidized andthe reduced forms of the redox couple must be separately considered,since the lower of the two values will limit the available currentdensity.

The redox potential of the redox couple should be, in the case of n-typesemiconductors, positive enough on the electrochemical scale to providea substantial barrier height with the conduction band of the n-typesemiconductor. For p-type materials, the opposite is true, i.e., theredox species should have a redox potential which is electropositive inorder to form a substantial barrier at the interface of the p-typesemiconductor interface. The redox potentials of many classes ofmolecules are known in tabulations available to those of ordinary skillin the art and may be also be predicted based on molecular calculationsor structural analogies to known molecules. Generally, for n-typematerials, redox couples which have redox potentials more positive than0.2 volts versus a saturated calomel electrode would be satisfactorywith the semiconductor. However, in the case of utilization with ann-type semiconductor if the redox couple is extremely oxidizable, it mayreact with the semiconductor in the dark as well as in the light,thereby causing corrosion.

Another property of the redox couple is that it should have a rate ofelectron transfer to the surface of the semiconductor rapid enough toallow significant current to flow. The theory which relates electrontransfer rates at surfaces with molecular electron exchange rates iswell known, such as described by R. A. Marcus, J. phys. Chem. 67, 853(1963). Many heterogeneous rates for redox molecules are tabulated.Therefore, if the rates of electron transfer to and from the redoxcouple to the respective electrodes are too slow, the current flux willbe limited not by the physical diffusion of the molecules to theelectrode surface or by photon flux, but by the probability that a givenredox molecule near the electrode surface will donate or receive chargeto electrode surface.

The preferred redox couples are ferrocene-ferrocenium couples; however,many redox couples may be selected which satisfy the above conditions.If the redox couple having an appropriate redox potential does not havethe requisite solubility in the solvent of choice, then the redox couplemay be chemically modified to impart greater solubility in the solvent.For example, the ferrocene molecule may be modified by introducing analcohol side chain thereto according to conventional methods to make itmore soluble in an alcohol solvent. In general, appropriate substituentsmay be introduced by known techniques to meet solubility requirements.Such substituents may include alkyl groups, carboxylic acids, esters,amides, alcohol groups, amino groups, substituted amino groups, sulfoxygroups, ketones, phosphate groups and the like. A preferred ferrocenerrocenium couple is dimethylferrocene [O]/dimethylferrocenium [+](DMFc/DFMc⁺) with an appropriate anion, such as tetrafluoroborate.

The solutes dissolved in the solution, i.e., the electrolyte and theredox species, should not appreciably absorb light in the wavelengthregions which induce photogeneration at the semiconductor surface, ifthe configuration of the cell requires that the light pass through thesolution in order to penetrate to the semiconductor. Generally, thesolutes should not absorb light above the semiconductor band gap energyfrequency or above wavelengths about 300 nm. In the case of siliconsemiconductors, the solutes should not absorb light between about 1100and 400 nm. If the solutes do absorb to an appreciable extent within anundesired region, proper concentration and solution thickness may beselected according to Beer's Law so that the absorbants do notsubstantially reduce efficiency of penetration of the light to thesemiconductor surface. For example, according to Beer's Law theabsorbance of a solution at a particular wavelength is defined by A=elcwhere e is the molar extinction coefficient at the chosen wavelength, 1is the path length of liquid in centimeters through which the light mustpass, and c is the concentration of the absorbing molecule inmoles/liter. Absorbance, A is related to transmittance, T, by therelationship log T=-A. Therefore, A of 0.1 is equivalent to atransmittance at that wavelength of 0.8 (80%), and if this is chosen tobe the minimum acceptable value, then to obtain A=0.1 with c=0.1 molar,el must equal 1 or less. Generally, the maximum values of e are in theorder of 10⁵, usually between 10² and 10⁴ for molecules which absorb inthe visible wavelengths. If a typical value of e is chosen at 10³ then 1should be less than 10⁻³ centimeters in order for the absorption by thesolute to be acceptable within the predetermined limit (80%).

To further enhance the efficiency of the photoelectrochemical cellsaccording to the present invention, it is desirable to etch a singlecrystal photoactive electrode surface to produce an increased surfacearea and to decrease reflectivity. Etching may increase the shortcircuit current by up to 50%. This treatment involves the use of etchsolutions according to known techniques which preferentially etchparticular crystal planes of the semiconductor, such as silicon.

For polycrystalline and amorphous semiconductors, etching may not benecessary. The first etch treatment of a single crystal semiconductormay also be followed by a second etch to remove oxide.

Referring to FIG. 2, there is shown a cross section of aphotoelectrochemical cell according to the present invention. Thesemiconductor photoactive electrode 10 (n-type, as shown) and ionicallyconducting solution 11 containing electrolyte and redox species arecontained within a nonconductive casing 12, such as glass. Electrolytesolution 11 is in contact with transparent counter-electrode 13. Atransparent sheet 14 overlays the cell and permits light to pass intothe cell to contact the semiconductor 10. Current collector 15 isconnected to lead 18. Leads 17 and 18 to the counterelectrode 13 andcurrent collector 15, respectively, may be connected to a load 19 toperform work, or to a battery to store energy generated by the cell.

Having described particular embodiments, the following examples are setforth by way of illustration of the present invention.

The following examples fall into two distinct categories: the twoelectrode and three electrode configurations. The two electrodeconfiguration presents a prototype working cell and is represented inFIG. 2. The three electrode configuration requires the use of areference electrode in addition to the working and counterelectrodes.The general theory of this configuration is described in ElectrochemicalMethods, Bard, A. J. and Falkner, L. F., John Wiley & Sons, N.Y. 1980.An electronic feedback system is employed to compensate any limitingreaction or overpotential which occurs at the counterelectrode so thatthe electrochemistry of the working electrode may be isolated. Thus, inthe case of an n-type semiconductor working electrode, the concentrationof the oxidized form of the redox species (reduction occurs at thecounterelectrode) may be artificially low. This configuration does notpresent a prototype working cell, but it is useful as a tool for study.And, as we will demonstrate in the following examples, a two electrodeconfiguration of the same system, designed with the rules set forthabove often leads to further increases in conversion efficiencies.

EXAMPLE 1 A 3-electrode LIS Cell With Single Crystal SiliconSemiconductor

Photoelectrochemical cells were constructed utilizing silicon electrodesfashioned from polished wafers of 1.3 to 1.7 ohm-cm resistivity,phosphorous-doped, (100) oriented single crystal material obtained fromMonsanto, Inc. Typical electrodes consisted of squares four millimeterson each side, rubbed on the unpolished side with Ga-In eutectic to forman ohmic contact. Shiny electrode surfaces were obtained by etching inconcentrated (48%) aqueous HF for 20 seconds followed by a methanolrinse. The sample was then contacted, using silver epoxy, to a copperwire threaded through a glass rod. The backside and the front perimeterof the sample were coated with insulating epoxy to define the electrodearea. Matte electrode surfaces were prepared by exposing unmountedsilicon to Transene Corp., Rolley, Massachusetts, Solar Cell EtchantType 200 for 60 minutes at 80° C., and were mounted as in the case forshiny surfaces. The matte electrodes were etched with 48% HF and rinsedwith methanol immediately before use. Methanol was Baker ChemicalCompany reagent grade, and was distilled under nitrogen from magnesiumpowder immediately before use. Lithium perchlorate, selected for itssolubility in methanol, was obtained from Alfa Ventron Inc. and was usedas received. Dry lithium perchlorate was obtained by fusion at 350° C.for 24 hours under 0.01 mm Hg vacuum and was stored in a dry box untiluse. Ferrocene was obtained from Aldrich Chemical Company, and waspurified by sublimation. Ferricenium ⁺ PF₆ ⁻ was prepared by the methodof Wahl, J. AM. Chem. Soc. 79, 2049-2052 (1975), the disclosure of whichis incorporated herein by reference in its entirety.(1-Hydroxy)ethylferrocene (hereinafter referred to as Fc -OH) wasprepared by reduction of acetylferrocene (Aldrich) with LiA1H₄ accordingto the procedure of Arimoto, J. AM. Chem. Soc. 77, 6295-6297 (1955), andwas dried in vacuo and stored under nitrogen. The ferricenium saltderivative (Fc⁺ -OH) was prepared by electrochemical oxidation inmethanol at +0.7 volts against a saturated calomel electrode (SCE) at alarge area platinum electrode. A Luggin probe was fashioned from aborosilicate glass pipette and had a measured outer diameter 0.2millimeters. The reference electrode (SCE) was connected to the cell bya salt bridge (1.0 M lithium perchlorate in methanol) to the Lugginprobe. For stability experiments, the SCE was replaced with a platinumwire reference electrode connected directly to the cell compartment. Thecounterelectrode was a large area (over 5 square centimeters) platinumelectrode. The conductivity of 1.0 M lithium perchlorate methanolsolution was measured to be 35 mmho/cm. The light source was an ELH-typetungsten halogen bulb with a ground glass diffuser.

A cell was assembled containing (100) oriented n-type silicon electrodesin 1.0 M lithium perchloratejmethanol with 0.2 molar Fc -OH, 0.5 mM Fc⁺-OH. The current voltage characteristic (50 mV/second) of the cell inresponse to light intensity of 70 mW/cm² from a calibrated solarsimulator ELH-type tungsten halogen source is shown in FIG. 4.Efficiency of 10.1% was observed for the matte surfaced cells and 7.8%for the polished surface cells. Using the same ingredients of this3-electrode cell, an analogous 2-electrode cell was constructed as setforth below in Example 2.

EXAMPLE 2 A 2-electrode LIS Junction Cell With Single Crystal SiliconSemiconductor

A thin cell was constructed in the following manner: the workingelectrode was fashioned from an n-type polished single crystal waferwith crystalline orientation (100) and resistivity 4-9 ohm-cm. First thesame was oxidized in wet oxygen at 1100° C. to form a dense oxide onboth sides of approximately 8000 Å in thickness. Then the perimeter ofthe sample was coated with positive photoresist to define an uncoatedsquare area slightly larger than one square centimeter. After baking at90° C. for 30 minutes (to harden the photoresist), the sample was etchedfor 12 minutes in semiconductor grade buffered oxide etch to remove theoxide in the defined square. Then the sample was treated in Transenesolar cell etchant type 200 in the manner described in Example 1 toprovide the defined area with a matte finish. An ohmic contact to thebackside was made as described in Example 1, and immediately prior touse, the sample was etched in 1:10 HF:H₂ O and rinsed in methanol.

The counterelectrode was fashioned with indium tin oxide (ITO) coatedglass obtained from OCLI having a sheet resistivity of approximately 10ohms per square and an integrated transmission of 85-90%. First, two 0.5mm holes were drilled in the ITO coated glass with an ultrasonic impactgrinder made by Raytheon Corporation. Then four 0.25 square centimeterstainless steel masks were placed on the sample such that they defined atotal area slightly larger than 1 square centimeter and equal to thearea previously defined on the silicon electrode. A 150 Å layer of Ti(for its adhesion properties) followed by a 1000 Å layer of Au (for itsconductive properties) were then evaporated in a filament evaporator ata pressure of 5×10⁻⁶ torr. The removal of the masks left four 0.25square centimeter transparent areas separated by two crossed grid linesof approximately 1 mm in width.

The working and counterelectrodes were then clamped together and gluedwith 5 minute epoxy. Two syringe needles were epoxied into the holes inthe ITO coated glass to provide a means for introducing the solution.

The contents of the solution were identical to those used in Example 1,however, the concentrations were modified to suit the two electrodeconfiguration using the conditions as set forth above. The solvent wasmethanol with 1.5M lithium perchlorate electrolyte, 0.12M Fc -OH and0.16M Fc⁺ -OH.

Contact to the cell was accomplished with two standard probes and thecell was illuminated through the ITO coated glass with 100 mW/cm²ELH-type illumination as calibrated with a Solarex silicon standard. Thecurrent-voltage characteristic is shown in FIG. 5, and the spectralresponse curve is shown in FIG. 6. A conversion efficiency of 12% (withno correction for reflection or solution absorption) was obtained. Fromthe Fc⁺ -OH absorption (1.9 eV) in the spectral response curve (FIG. 6),the thickness of the liquid in the cell was deduced to be approximately20 microns.

EXAMPLE 3

A 3-electrode Cell with n-GaAsP Epilayer Semiconductor

A semiconductor photoanode was prepared of the formula n-GaAs₀.72 P₀.28obtained as an epilayer deposited on n⁺ GaAs or n⁺ GaP substrate byvapor phase epitaxial techniques. The layer thickness was large enough(0.1 mm) such that no photoelectrochemical effects would be ascribableto the substrate material. Samples (0.1 cm²) were mounted asphotoelectrodes by forming ohmic contacts to the substrates byevaporating In at 1×10⁻⁶ torr and annealing under nitrogen at 400° C.for 15 minutes. Samples were attached to a copper wire with silver paintand insulated with epoxy cement. Light sources utilized were eithersunlight or a calibrated W-halogen ELH lamp with a ground glassdiffuser. The samples were etched with 1:1 HF/H₂ O₂ for 15 seconds,rinsed with H₂ O and air dried before use.

W-halogen irradiation (88 mW/cm²) of a sample of n-GaAs₀.72 P₀.28 (notdeliberately doped; N_(D) =3.3×10¹⁵) in a solution of 0.1M ferrocene(Fc), 0.5 mM ferricenium (Fc⁺) in dry acetonitrile (ACN) solvent(electrolyte is 1.0 M (C₂ H₅)₄ N⁺ BF⁻ ₄) produced an observed opencircuit voltage of 1.01 volts and a short circuit current of 15.7 mA/cm²with a fill factor of 0.73, leading to an optical-to-electricalconversion efficiency of 13.2% (15.2 mA/cm² and 0.76 volt at the pointof maximum power). In natural sunlight (64 mW/cm²), we observe similarbehavior, and conversion efficiencies of 12.5 to 13.0%.

EXAMPLE 4 A 3-electrode Cell With n-GaAs Semiconductor

Photoelectrochemical cells were prepared utilizing n-GaAs semiconductorelectrodes in 0.1M ferrocene (Fc)/0.5 mM ferrocenium (Fc⁺) inacetonitrile and 1.0M [(C₂ H₅)₄ N]⁺ [BF₄ ⁻ ]. The n-GaAs sampes (100)oriented, were etched with 1:1 H₂ SO₄ :H₂ O₂ to a matte finish andmounted in a conventional cell as described above in connection withExample 3. Under 88 mW/cm² of ELH-type irradiation, an open circuitvoltage of 0.7 volts with a short circuit current of 24 mA/cm²,resulting in 9.5% efficiency for conversion of light to electricity,were observed. In direct sunlight similar behavior was observed and atirradiation levels of 65 mW/cm² 10.0% was observed to be the conversionefficiency of solar radiation into electricity.

EXAMPLE 5 A 3-electrode Cell With Single Crystal p-type Silicon

Electrodes were fashioned from 4-9 ohm-cm boron doped (100) orientedsingle crystal silicon obtained from Siltec. The preparation wasidentical to that described in Example 1 except that ohmic contact wasmade by evaporating 2000 Å of aluminum and sintering at 650 degreescentigrade for 15 minutes. A solution of 0.20Mbis(cyclopentadienyl)cobalt perchlorate and 0.5 mM bis(cyclopentadienyl)cobalt was dissolved in acetonitrile with 1Mtetraethylammonium boron terafluoride as electrolyte. The redoxpotential of the solution was -0.82% volts vs. SCE, as measured througha Luggin capillary vs. a platinum foil electrode.

A sample, mounted and matte textured as described in Example 1 andmeasured to be 0.143 square centimeters in area, was etched in 48% HFfor 15 seconds, rinsed with water, then with acetonitrile, air dried andinserted into a three electrode cell fitted with a Luggin capillary asdescribed in Example 1. Under 70 mW/cm² of ELH illumination, we observean open circuit voltage of 0.49-0.53V, a short circuit current of 23-26mA/cm² and a conversion efficiency of 10.5%. This result may be improvedin a two electrode configuration similar to that described in Example 2.

EXAMPLE 6 A 3-electrode LIS Cell With Amorphous Silicon Semiconductor

A layer of amorphous silicon was deposited by rf plasma decomposition ona stainless steel substrate. A 100 Å layer of phosphorous dopedamorphous silicon followed by a 0.5 micron layer of intrinsic amorphoussilicon characterized the deposition. The backside was contacted with acopper wire threaded through a glass rod. An electrical connection tothe backside was made with silver epoxy and then the electrode wasattached to the rod with insulating epoxy. At the same time, thebackside and the perimeter of the front side were also covered withinsulating and opaque epoxy to define the electrode area. The area wasmeasured to be 0.28 square centimeters.

The solution used in this cell had the same ingredients as described inExample 1. The solution was methanol with 1M lithium perchlorate and theredox pair was 0.15M Fc-OH, 0.5 mM Fc⁺ -OH. A Luggin capillary was usedto minimize uncompensated resistance. At 70 mW/cm² ELH-type illuminationthere was observed a short circuit current of 2.7 mA/cm² and an opencircuit voltage of 0.72 volts with a conversion efficiency of 1.3%. Thelow current values were attributed to solution absorption and to thetransport properties of these amorphous silicon samples.

EXAMPLE 7 A 2-electrode Cell With n-type Amorphous Silicon Semiconductor

A thin layer of amorphous silicon was deposited by rf plasmadecomposition on a degenerately doped n-type single crystal siliconsubstrate. A 100 Å layer of n+ amorphous silicon followed by a 0.5micron layer of intrinsic amorphous silicon characterized thedeposition. An ohmic back contact was accomplished as in Examples 1 and2 with Ga-In eutectic. The counter-electrode was fashioned with ITOcoated glass supplied by OCLI having the characteristics described inExample 2. As in Example 2, first two holes were ultrasonically drilledin the ITO coated glass. Then the glass was masked with a square siliconslice of area 0.065 square centimeters. The masked glass was placed in afilament evaporator and, at a system pressure of 5×10⁻⁶ torr, 5000 Å ofaluminum was evaporated onto it. A larger square silicon mask of area0.15 square centimeters was then placed on the ITO coated glass and theregion surrounding the mask was coated with an 8000 Å insulating film ofchemical-vapor-deposited SiO₂. The counter-eletrode and the workingelectrode wer then clamped together and epoxied (as in Example 2). Thesolution, identical to the one Example 2, was introduced by means ofsyringes which were epoxied to the previously drilled holes.

A conversion efficiency of 3.1% was observed under 100 mW/cm² ofELH-type irradiation. As shown in FIG. 7, there was observed an increasein efficiency after the cell was held at maximum power for approximately1.5 hours. Open circuit voltage increased from 0.77 volts to 0.85 voltswhile short circuit current increased from 6.9 mA/cm² to 7.2 mA/cm². Thecorresponding increase in efficiency was from 3.1% to 3.5%. The increasein efficiency was attributed to the formation of an optimized thicknessof oxide on the working electrode surface which creates the optimizedLIS structure discussed in this application. It was noted that thecharacteristics of the amorphous silicon used were less than optimal. Itis expected that a reflecting substrate and an optimized thickness willyield a substantial improvement in conversion efficiency.

EXAMPLE 8 A 2-electrode Cell With n-type Polycrystalline SiliconSemiconductor

Samples of n-type cast polycrystalline silicon were obtained from WackerSiltronic. The samples were characterized by a resistivity of 1-3 ohm-cmand a preferred crystal orientation of (100). They were diced into onesquare centimeter squares with a diamond saw, then treated for 1.5 hoursin the Transene matte etch under the conditions described in Example 1.Following the matte etch a sample was further etched in 10:1 H₂ O:HF for10 minutes and rinsed in H₂ O. A square mask of approximate area 0.36square centimeters was then placed in the center of the sample and thesample was covered with a 7000 Å insulating layer ofchemical-vapor-deposited SiO₂. Upon removal of the mask, a workingelectrode with a bare silicon area surrounded by an insulating oxide wasobtained. Ohmic contact to the backside was made with gallium indiumeutectic as in Example 2.

The counterelectrode was fashioned with ITO coated glass in the generalmanner described in Example 2; however, only one masking square wasused, and the resulting cell area was therefore 0.25 square centimeters.

After a 30 second etch in 10:1 H₂ O:HF followed by a methanol rinse theworking electrode was clamped to the counterelectrode and epoxied. As inExample 2, syringe needles were epoxied to holes in the ITO coatedglass, and the solution was introduced. The solution was identical tothe one described in Example 2.

In FIG. 8, the current voltage characteristics of this cell arepresented. Under 100 mW/cm² of ELH illumination a conversion efficiencyof 7.8% was observed.

EXAMPLE 9 A 2-Electrode LIS Junction Cell With Single Crystal SiliconSemiconductor

A cell was constructed in a manner similar to that described in Example2 except that: silicon bulk resistivity was optimized to 0.3 ohm-cm(from 4-9 ohm-cm); the cell area was 0.25 square centimeters; ohmiccontact was formed by exposing the back surface to POCl₃ diffusion at950 degrees centigrade followed by an evaporation of a conductivealuminum layer; and the solution was 0.15M DMFc/0.15 M DMFc⁺ BF₄ ⁻ /1.0MLiClO₄ /methanol. The open circuit voltage was 0.61 volts and energyconversion efficiency of 14% was obtained.

EXAMPLE 10 Stability Comparison Between Cells Utilizing PropyleneCarbonate/Octanol Solvent vs. Methanol Solvent

A cell (Cell A, FIG. 9) was constructed in a manner according to Example2 except that: the Ti/Au on the ITO was etched using standardphotolithographic techniques to form a 4 square centimeter cell areawith a grid pattern which lowered the sheet resistivity (the gridcoverage was about 10%); the silicon was 0.3 ohm-cm n-type materialprepared as used in Example 9; the solution was well purified 0.64MDMFc/0.17M DMFc⁺ BF₄ ⁻ /0.5M LiClO₄ /propylene carbonate with 15%octanol by weight. The solution was purified by the following steps:

(a) stored solution over Linde 3Å and 4Å sieves

(b) freeze-pump-thawed solvents

(c) recrystallized redox couple several times

(d) purified redox couple with sublimation techniques

(e) fused LiClO₄ to eliminate water contamination

Water concentration in the solution was estimated to be less than 10 ppmby Karl-Fisher titration on similarly prepared solution constituents.

Stability tests were conducted in a Vacuum Atmosphere Company dry boxfor over one month. Maximum efficiency for Cell A in FIG. 9 was 10%.Cell A retained over 90% of its maximum efficiency after a month ofcontinuous operation at maximum power point under 100 mW/cm2 incidentillumination. This performance compares favorably to that obtained witha similarly prepared methanol based Cell B.

EXAMPLE 11 Cell With Propylene Carbonate/Methanol Solvent

A cell similar to Cell A in Example 10 was prepared except that thesolution consisted of 0.25M DMFc/0.25M DMFc⁺ BF₄ ⁻ /1.0M LiClO₄/propylene carbonate with 10% methanol by volume. Energy conversionefficiency of 13% was obtained.

EXAMPLE 12 A 2-Electrode Cell With n-type Polycrystalline SiliconSemiconductor

Samples of n-type polycrystalline silicon were obtained from WackerSiltronic. The samples were characterized by a resistivity of 1-3 ohm-cmand a preferred crystal orientation of (100). They were diced into onesquare centimeter squares with a diamond saw, then treated as follows:

(1) 12 minute etch in 10:4:1:10 nitric:acetic:hydrofluoric acid:DIwater;

(2) Phosphorous diffusion both sides 950 degrees centigrade;

(3) deposit oxide back side and anneal;

(4) 35 minute etch in 10:1 nitric:hydrofluoric acid (until shiny);

(5) 30 second dip in 10:1 DI water:hydrofluoric acid;

(6) 20 minute etch Transene Solar Cell Etchant 200;

(7) 10 minute etch 10:1 DI water:hydrofluoric acid;

(8) Deposit oxide front side to define 0.25 square centimeter area;

(9) Evaporate Cr/Ag to backside for ohmic contact.

A cell was prepared using these wafers as described in Example 8, exceptthat the solution was 0.3M DMFc/0.3M DMFc⁺ BF₄ ⁻ /1.0M LiClO₄ /methanoland the ITO was prepared as described in Example 10. Cell efficiency was9.6%.

What is claimed is:
 1. A method for converting solar energy toelectrical energy, comprising the step of exposing to solar radiation acell comprising a semiconductor photoactive electrode, a nonaqueoussolvent and an electrolyte dissolved therein forming an ionicallyconductive solution, a counterelectrode providing an electricalconnection from said solution to said photoactive electrode; and a redoxcouple dissolved in said solvent suitable to accept and donate electronsfrom and to said electrodes upon incidence upon said cell by an externalsource of light, said redox couple present in an amount sufficient tosustain a predetermined current; wherein the concentrations of saidelectrolyte and said redox couple in said solution are selected toprovide no greater than a predetermined small voltage drop in comparisonto the output voltage of said cell during conversion in said cell ofsaid light to electricity.
 2. A method according to claim 1 wherein saidsolvent further comprises an agent which induces formation of apredetermined amount of an insulating compound at the interface of saidsemiconductor electrode and said solution.
 3. A method according toclaim 2 wherein said agent is water and said semiconductor comprisessilicon.
 4. A method according to claim 1 wherein said semiconductorconductor is an n-type.
 5. A method according to claim 1 wherein saidsemiconductor is a p-type.
 6. A method according to claims 4 or 5wherein said solution is a liquid film of a thickness less than 100microns.
 7. A method according to claims 4 or 5 wherein the band gap ofsaid semiconductor is in the range of 1.0 to 2.3 eV.
 8. A methodaccording to claim 7 wherein said semiconductor is selected from thegroup consisting of silicon, gallium arsenide, gallium arsenidephosphide, cadmium telluride, cadmium selenide, indium phosphide, anda-Si:H alloy.
 9. A method according to claim 8 wherein saidsemiconductor is selected from silicon, GaAs and GaAs_(x) P_(1-x),wherein x is between 0.0 and
 1. 10. A method according to claim 8wherein said semiconductor comprises cadnium telluride.
 11. A methodaccording to claim 4 or 5 wherein said semiconductor is amorphous.
 12. Amethod according to claim 5 wherein said semiconductor comprisesp-silicon.
 13. A method according to claim 1 wherein said nonaqueoussolvent is characterized by a dielectric constant greater than 20 and aviscosity less than 2 centipoise.
 14. A method according to claim 13wherein said solvent is selected from an alkanol of 1 to 10 carbon atomsor nitrile of 2 to 10 carbon atoms.
 15. A method according to claim 14wherein said solvent is selected from methanol or acetonitrile.
 16. Amethod according to claim 13 wherein said electrolyte is selected fromlithium perchlorate and tetra alkyl ammonium borontetrafluoride.
 17. Amethod according to claim 1 wherein said redox couple is selected fromferrocene-ferrocenium salts and derivatives thereof.
 18. A methodaccording to claims 4, 16, 15 or 17 wherein said electrolyte is lithiumperchlorate, said solvent is methanol, said redox couple is(1-hydroxyethyl) ferrocene/ (1-hydroxyethyl) ferrocenium salt, and saidsemiconductor is n-type silicon.
 19. A method according to claims 4, 16,15 or 17 wherein said electrolyte is tetraethyl ammoniumborotetrafluoride, said solvent is acetonitrile, said redox couple isferrocene/ferrocenium salt and said semiconductor is an epitaxial layerof GaAs₀.72 P₀.28 on a GaAs substrate.
 20. A method according to claims4, 16, 15 or 17 wherein said solvent is acetonitrile, said redox coupleis ferrocene/ferrocenium salt, said electrolyte is tetraethyl ammoniumborontetrafluoride and said semiconductor n-GaAs.
 21. A method accordingto claim 1 wherein the selected small voltage drop is less than 10% ofthe output voltage.
 22. A method according to claim 1 wherein thepredetermined current is the solar photon flux with energies greaterthan the bandgap of the photoactive electrode multiplied by the chargeof an electron.
 23. A photoelectrochemical cell having improved solar toelectrical energy efficiency in the range of 10% and above, comprising asemiconductor photoactive electrode, a nonaqueous solvent and anelectrolyte dissolved therein forming a ionically conductive solution, acounterelectrode providing an electrical connection from said solutionto said photoactive electrode; and a redox couple dissolved in saidsolvent suitable to accept and donate electrons from and to saidelectrodes upon incidence upon said cell by an external source of light,said redox couple present in an amount sufficient to sustain apredetermined current; wherein the concentrations of said electrolyteand said redox couple in said solution are selected to provide nogreater than a predetermined small voltage drop in comparison to theoutput voltage of said cell during conversion in said cell of said lightto electricity, and wherein said solvent comprises a mixture of alkylenecarbonate and an alcohol of 1-10 carbon atoms and said redox couple is aferrocene-ferrocenium salt or derivative thereof.
 24. A cell accordingto claim 23 wherein said solvent comprises propylene carbonate.
 25. Acell according to claim 24 wherein said redox couple comprisesdimethylferrocene-dimethylferrocenium salt.
 26. A cell according toclaim 25 wherein said solvent comprises an alcohol selected from thegroup consisting of n-octanol, n-hexanol, n-butanol, trifluoroethanol,and methanol.
 27. A cell according to claim 26 wherein said alcohol ismethanol.
 28. A cell according to claim 26 wherein said alcohol isn-octanol.